What project owners need to know.
The Siemens process is the chemical route that turns metallurgical-grade silicon into high-purity polysilicon: the silicon is converted into the liquid trichlorosilane (SiHCl₃), purified by distillation, and then reduced with hydrogen at about 1,100 °C so that pure silicon deposits on heated silicon rods. Its by-products, hydrogen, hydrogen chloride and silicon tetrachloride, are recovered and recycled, which is why it is best understood as a loop rather than a line.
Why polysilicon needs chemistry, not just a furnace
Silicon starts as quartz. PVEducation describes quartz being reduced with carbon in an electrode arc furnace at 1,500–2,000 °C to make metallurgical-grade silicon (MG-Si), which is only about 98% pure and still carries metals and hundreds of parts per million of boron and phosphorus. That is fine for aluminium alloys and silicones, but far too dirty for a wafer.
No furnace can lift silicon from about 98% to the purity a solar or chip wafer needs. The answer is to stop treating it as a solid for a while. The Siemens process turns silicon into a liquid compound, purifies that liquid, and turns it back into silicon. Siemens developed the route in the 1960s, and it still makes most of the world's polysilicon.
- Quartz → MG-Si: carbothermic reduction in an arc furnace
- MG-Si: about 98% silicon, with metals, boron and phosphorus
- Siemens route: silicon → trichlorosilane → distillation → silicon
Step 1: Making trichlorosilane
Ground MG-Si reacts with hydrogen chloride (HCl) in a fluidised bed at about 300 °C. The main product is trichlorosilane (SiHCl₃, or TCS), with some silicon tetrachloride (SiCl₄, or STC) and hydrogen. Fraunhofer ISE gives the overall balance as roughly 0.9 parts TCS to 0.1 parts STC.
Impurities such as iron, aluminium and boron also react and form their own chlorides. That is the point of the step: once everything is a chloride, it can be separated by boiling point.
Step 2: Distillation, where the purity is made
TCS boils at 31.8 °C and STC at 56.8 °C, while the impurity chlorides boil at other temperatures. Multi-stage distillation separates them. PVEducation puts electrically active impurities in the purified TCS below one part per billion.
Fraunhofer ISE notes that electronic-grade polysilicon typically uses about five distillation cycles against three for solar grade. The heat for distillation can largely come from recovered heat from the deposition reactors.
- TCS (SiHCl₃): boils at 31.8 °C
- STC (SiCl₄): boils at 56.8 °C
- Solar grade: about 3 distillation cycles; electronic grade: about 5 (Fraunhofer ISE)
Step 3: Growing the rods
Purified TCS and hydrogen flow into a bell-shaped chemical vapour deposition (CVD) reactor. Inside, thin silicon cores, often a U-shape held between graphite electrodes, are heated electrically to around 1,100 °C. TCS is reduced on their surface and pure silicon builds up layer by layer.
Fraunhofer ISE describes slim rods starting at about 20 mm and finishing at 150–180 mm in diameter, with growth rates of up to about 1.9 mm per hour for solar grade and closer to 1 mm per hour for electronic grade, where uniform growth matters more. The rods are then crushed into chunks and etched, which is the form most ingot plants load into a crucible.
- Bell-jar CVD reactor with electrically heated silicon cores
- Rods grow from ~20 mm to 150–180 mm diameter
- Crushing and acid or alkaline etching before packing
Step 4: The loop that makes it viable
Deposition is not complete conversion. The off-gas carries hydrogen, HCl, unreacted TCS and STC. Fraunhofer ISE describes condensation units separating them: hydrogen goes back to the reactor, HCl goes back to TCS synthesis, and the chlorosilanes are distilled again.
STC is the by-product to watch. It can be converted back towards TCS in a modified Siemens route, or sold as a precursor for silicones and silica products. A plant that cannot handle its STC has a chemical-handling problem as well as a yield problem, and this is one reason polysilicon is a chemical-plant project rather than an extension of a wafer line.
- Hydrogen → back to the CVD reactor
- HCl → back to TCS synthesis
- STC → converted back towards TCS, or sold as a chemical precursor
Solar grade versus electronic grade
The chemistry is the same; the discipline differs. Fraunhofer ISE's comparison, adapted from the Chinese standard GB/T 12963-2022, shows electronic grade allowing far fewer donor and acceptor atoms than solar grade, and it uses smaller reactors, longer cycles, more distillation and more surface etching to get there.
Electricity is the main resource. Fraunhofer ISE puts solar-grade polysilicon at 60–71 kWh per kg for 2023, with about 60% of the power going to deposition, and estimates electronic grade at about 92–124 kWh per kg for 2025. These are industry-level resource figures, not plant costs, and any specific plant will differ.
- Same TCS route for both grades
- Electronic grade: tighter dopant limits, slower and smaller reactors
- Solar grade: 60–71 kWh/kg (2023); electronic grade: ~92–124 kWh/kg (2025 estimate)
What this means for India
India has built module and cell capacity quickly, but polysilicon is the stage it has not yet started. ThePrint reported on 20 August 2026 that India still produces no polysilicon, that a few groups have announced plans, and that MNRE said on 7 August 2026 it was planning a production-linked incentive for polysilicon whose details had not been released.
The process explains why this stage is slow. It needs large, reliable power, handling of chlorine compounds and hydrogen, multi-stage distillation and a closed recovery loop, run continuously for years. It also starts with quartz and MG-Si, which ties polysilicon back to India's high-purity quartz question. For ingot makers preparing for ALMM List-III, the near-term point is simpler: specify the polysilicon grade and form you need, and ask suppliers for documented purity and origin.
- Commercial polysilicon production: not yet started in India (ThePrint, Aug 2026)
- Polysilicon PLI: announced as planned, details not released (Aug 2026)
- Ingot makers: specify grade, form and documented origin of feedstock
How JRST supports this requirement
JRST works on India's upstream silicon chain, from high-purity quartz and crucibles to ingot growth and wafering, and helped deliver Maharashtra's first silicon-ingot pilot. This guide describes the generic, publicly documented Siemens process for awareness only. It is not a design basis or an economic assessment: any polysilicon project's process, energy and safety figures must be validated with licensors and qualified engineers.
Discuss your requirement
Mohammed Saif Zaveri connects JRST's industrial content to execution conversations.
As Co-Founder and designated partner of JRST Technology LLP, Mohammed Saif Zaveri works across industrial growth, strategic partnerships, client conversations, equipment strategy, and project pathways for silicon, solar, semiconductor, and advanced-manufacturing opportunities.
This knowledge page is part of JRST's public industrial knowledge base, designed to help buyers move from search terms and early research toward a structured technical-commercial discussion.
View Mohammed Saif Zaveri's profileConnect on LinkedInFrequently asked questions
What is the Siemens process?
The Siemens process is the main industrial route to high-purity polysilicon. Metallurgical-grade silicon is turned into trichlorosilane, purified by distillation, and reduced with hydrogen at about 1,100 °C so that pure silicon deposits on heated silicon rods.
What is trichlorosilane used for?
Trichlorosilane (SiHCl₃) is the liquid intermediate of the Siemens process. Because it boils at 31.8 °C, it can be purified by distillation and then reduced back to very pure silicon.
What is silicon tetrachloride in polysilicon production?
Silicon tetrachloride (SiCl₄) is the main by-product of the trichlorosilane route. It is recovered and either converted back towards trichlorosilane or used as a precursor for silicones and silica products.
How much electricity does polysilicon production use?
Fraunhofer ISE puts solar-grade polysilicon at 60–71 kWh per kg in 2023 and estimates electronic grade at about 92–124 kWh per kg in 2025, with deposition the largest share. Individual plants vary.
What is the difference between solar-grade and electronic-grade polysilicon?
Both are usually made by the same Siemens chemistry, but electronic grade has much tighter limits on dopants, carbon and metals and needs more distillation, slower deposition and more etching.
Does India make polysilicon?
Not yet at commercial scale. ThePrint reported in August 2026 that India produces no polysilicon and that MNRE was planning a polysilicon incentive scheme whose details had not been released.
How is the Siemens process different from FBR granular polysilicon?
The Siemens process deposits silicon on heated rods that are later crushed into chunks. A fluidised bed reactor (FBR) deposits silicon on small seed particles to make granules, typically from silane, with lower electricity use but different handling issues.
How can JRST help?
JRST does not make polysilicon. It works downstream of it, on ingot growth, crucibles, hot zones and wafering, and helps buyers frame polysilicon grade and form requirements for their crystal growers.
Primary sources and further reading
Last reviewed 2026-10-14. Technical scope, policy eligibility, availability, and commercial terms should be independently confirmed for each project.